score = float(input('请输入一个百分制成绩：'))
if score < 0 or score > 100:
    print('无效成绩')
else:
    score = score // 10
    if score in (9, 10):
        print('A')
    elif score == 8:
        print('B')
    elif score == 7:
        print('C')
    elif score == 6:
        print('D')
    else:
        print('F')